i) Field of the Invention
This invention relates to a filler treatment process, an aqueous filler composition and a treated filler, and a pulp furnish, all for use in paper manufacture; and to a method of making paper and to a paper.
ii) Description of the Prior Art
In the manufacture of filled paper and paperboard grades, filler slurries at consistencies ranging from 10 to 70% are added to pulp furnishes before the web forming section of the paper machine. The papermaker may also add other additives, such as a natural and synthetic polymeric strength agent, a sizing agent, alum, dyes, a fluorescent brightening agent and a retention aid system. The retention aid system is always added to the final furnish prior to the headbox to retain as much of the filler as possible in the sheet.
Filler contents up to 25% are typical of current papermaking where the filler improves the optical properties of the paper such as brightness and opacity as well as improving the feel of the sheet and the print quality of the printed sheet. In some instances, the economics of replacing expensive fibre with inexpensive filler lends added incentive to increase the amount of filler in paper. The savings can be substantial when low cost fillers, such as kaolin clay, precipitated calcium carbonate (PCC), ground calcium carbonate (GCC), chalk, talc, or precipitated calcium sulphate (PCS), are used to replace expensive pulp fibres. Moreover, filled paper is much easier to dry than paper with no filler and, as a result, the papermachine could run faster with less steam consumption, which reduces energy costs and improves productivity. Therefore, the replacement of a fraction of fibre by filler in paper could significantly reduce the production cost of paper.
For a given sheet grammage there are, however, limits to the amount of filler that can be added to the pulp furnish. The strength of paper and its printing properties (printability) are usually the most important factors limiting the filler content in paper, although other factors, such as papermachine runnability, retention, drainage, formation, dusting and sizing, are also a consideration.
In general, no matter how strong the pulp fibres and their bonding in paper is, all common fillers (e.g., clay, GCC, PCC, chalk, talc, PCS) are known to impair significantly all paper strength properties, including internal bond strength, surface strength, tensile, burst, tear, and stiffness. For example, it has been found that for each 1% filler addition to paper sheet the loss in tensile strength can range between 1 and 3%, depending on the type of pulp furnish. Sheet strength is inevitably reduced since a portion of fibres have been replaced by filler; not only because there are fewer fibres in the sheet, which reduces the number of fibre-fibre bonds, but also because the presence of the filler decreases the area of contact and prevents hydrogen bonding from occurring between the remaining pulp fibres. As a result, making a fibrous web with a high amount of filler produces a weaker sheet that can break more easily on the paper machine, size press, coater, winders and printing presses. Weaker fibre-fibre bonding also decreases the surface strength of the paper, causing a reduction in pick resistance and a tendency for increased linting. Poor bonding of filler particles in the fibrous structure, especially those located at the sheet surface, can increase dusting and piling in the pressroom and during converting.
Sizing chemicals, such as alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA), are added to pulp furnishes in order to increase the hydrophobicity of the fibre and thus reduce water and liquid penetration into the sheet. In general, calcium carbonate fillers are known to increase the amount of sizing chemicals required for internal sizing paper. In particular, scalenohedral PCC, which is widely used in the manufacture of fine papers, produces excessive negative effects on sizing, which increases significantly the size chemical demand for maintaining target sizing value. As the content of PCC is increased in the furnish the demand for sizing chemicals is increased to maintain the desired degree of sizing or water repellency. Poor sizing efficiency and loss of sizing over time (size reversion) are common problems associated with the PCC-filled fine papers. Poor sizing affects liquid penetration and can be detrimental for coating and printing.
The retention of filler during web forming, even when assisted by retention aid chemicals, is often a major problem with all paper grades, especially for high speed machines and in the manufacture of light-weight and highly-filled grades. Since filler retention during sheet making is never 100%, as the filler content in pulp furnish is increased to 30-70% of the pulp fraction the filler concentration in the whitewater will significantly increase. In many paper mills machine runnability problems, paper defects, increased filler losses, and increased chemicals cost have been associated with high white water ash consistency. With common retention aid chemical systems it is possible to achieve high filler retention in paper by increasing the dosage of chemicals, but this is difficult to do without impairing web formation due to over-flocculation of furnish components. Therefore, a method that improves filler retention without excessive flocculation is required.
An ongoing industry trend is to decrease sheet grammage to reduce furnish costs. However, when the grammage is decreased nearly all paper properties deteriorate, including the limiting factors of opacity, stiffness and permeability. To overcome the loss in opacity due to basis weight reduction the papermaker can add expensive opaque pigments (e.g., titanium dioxide, calcined clay, sodium silicates or organic pigments) but this in turn can cause further deterioration in sheet strength. Reduction in grammage also decreases the retention of filler and increases the frequency of sheet breaks both on the paper machine and during converting and printing. Reducing sheet grammage may also lead to increased demand for sizing to control liquid absorbency.
A common method for improving the strength of filled paper and paperboard grades is the addition of high molecular weight polymers to pulp furnishes, such as cationic starches or cationic synthetic polymers. While the adsorption of a cationic polymer on naturally anionic pulp fibres can improve inter-fibre bond strength in paper, the presence of fillers will still cause de-bonding between fibres. Another limiting factor for the performance of cationic polymers is the presence of anionic dissolved and colloidal substances (DCS) in the furnish. These anionic DCS generally deactivate a large portion of the cationic polymer added making it less effective for bonding fibres. Anionic polymers can be used as a replacement for cationic polymers, but these polymers do not readily adsorb on anionic pulp fibres. To improve their retention on anionic fibres the addition of a cationic agent such as alum or synthetic polymer is required.
Mechanical pulp papers, including newsprint, groundwood specialties and supercalendered grades, have traditionally been made with clay fillers under acidic conditions. Although the addition of calcium carbonate fillers can improve the brightness and opacity of these papers at low cost, these fillers are still not widely used, because of the alkalinity of calcium carbonate. Mechanical pulp is usually weakly acidic, but if calcium carbonate is added to the pulp stock the pH will rapidly rise to above pH 8, causing the lignin in the mechanical pulp fibres to darken. The brightness drop of mechanical pulps due to a change in pH from 5 to 9 varied between 1.7 and 7.8 points, depending on the type and nature of pulp used [Evans, D. B., Drummond D. K., Koppelman M. H. “PCC fillers for groundwood papers”. 1991 Papermakers Conference, TAPPI Proceedings, p 321-330]. Thus, to minimize darkening, paper made from mechanical pulp should suitably be made under slightly acidic (pH 6.5) or neutral conditions (pH 7.0). However, in the presence of acid, calcium carbonate dissolves to produce calcium ions and carbon dioxide gas. To apply calcium carbonate filler in wood-containing grades the calcium carbonate filler must remain stable under weakly acidic or neutral pH conditions. In recent years many paper mills making wood-containing grades have converted to neutral papermaking to allow the use of bright calcium carbonate fillers (GCC and PCC), but the stability of CaCO3 filler at neutral pH and the amount of acid required to maintain neutral pH still remain major concerns. A method that makes calcium carbonate resistant to acid would allow mechanical pulp paper to be produced with PCC or GCC under neutral conditions.
The above information suggests that the paper industry needs cost-efficient technology for the production of highly-filled grades with good filler retention, drainage and formation, and acceptable strength, optical, and printing characteristics. A method that can make the filler particles adhere to themselves and to fibres without causing too much de-bonding between fibres may allow the papermaker to efficiently use polymers for strengthening filled papers. Furthermore, the filler should be stable at neutral pH so it can be used in the production of wood-containing grades.
In the industry, different water-based anionic polymer latex dispersions (such as styrene-butadiene, acrylate-styrene, acrylate-styrene-acrylonitrile, styrene-butadiene-acrylonitrile, acrylate-vinyl acetate) are added to various pigments in order to achieve many objectives, for example, in paint formulations where the latex increases storage stability and pigment compatibility. The use of polymer latex dispersions followed by water evaporation is a very convenient technique for obtaining uniform rubber films. The film formation process has three steps. First, the water evaporates, whereby the latex particles comes into contact with each other, then deformation of the latex spheres occurs and, finally, these deformed polymeric particles coalescence resulting in a uniform and continuous film. Furthermore, polymer latex dispersions are also widely used in paper coating formulations as a binder for fillers and pigments. The lower the glass transition temperature (Tg) of the latex the lower is the minimum film-forming temperature.
Anionic polymer latex dispersions do not readily adsorb on pulp fibres and, thus, are not used alone as paper making furnish additives. However, it is known in the paper industry that the addition of anionic latex followed by the addition of alum causes the latex particles to precipitate onto pulp fibres. Due to their small size and high surface areas the latex particles can cover a large surface area of pulp fibres. The presence of such latex in the paper sheet can act as a binder after drying and thereby give increased strength to paper and paper board products. Cationic polymer latex dispersions, which can readily adsorb on pulp fibres, are not commonly used as furnish additives probably due to their high cost.
Another approach for improving filler retention, strength and sizing performance is by treating the filler slurry with additives prior to mixing it with the pulp stock. For example, several patents, including U.S. Pat. No. 4,225,383, U.S. Pat. No. 4,115,187, U.S. Pat. No. 4,445,970, U.S. Pat. No. 5,514,212, GB 2,016,498, U.S. Pat. No. 4,710,270, and GB 1,505,641, describe the benefits of filler treatment with additives on retention and sheet properties. It is known that since most common inorganic filler particles in suspension carry a negative charge, the cationic additive adsorbs on their surfaces by electrostatic interactions causing them to agglomerate or flocculate. For anionic additives to promote flocculation the filler particles would require a positive charge to allow adsorption of the anionic additive. The aggregation of filler particles improves retention during sheet making and can also decrease the negative effect of filler on sheet strength, but excessive filler aggregation can impair uniformity and also decrease the gain in optical properties expected from the filler addition.
GB 1,505,641 discloses treating positively charged chalk whiting (natural ground calcium carbonate) with anionic styrene-butadiene (SB) latex dispersions. The filler particles are made cationic by the addition of the cationic starch with the objective to promote the adsorption of the anionic SB latex on the surfaces of filler particles. The preferred SB latex of GB 1,505,641 has at least 60% of its units derived from styrene. Treatment of cationic calcium carbonate filler, especially chalk whiting, with this SB latex is used to produce protected filler particles, which are then added during papermaking to improve the strength of the filled sheet. The latex-treated cationic chalk whiting slurry, containing up to 20 parts of latex per 100 parts of cationic chalk, is added before the headbox of the paper machine, for example, to the beater or pulper.
In U.S. Pat. No. 7,074,845B2 anionic latex has been used in combination with swollen starch for preparing treated filler slurries to be added internally in paper manufacture. The swollen starch/latex compositions are prepared by pre-mixing latex with a slurry of starch granules in a batch or jet cooker, or by adding hot water to the mixture under controlled conditions in order to make the starch granules swell sufficiently to improve their properties as a filler additive but avoid excess swelling leading to their rupture. The anionic latex interacts with cationic swollen starch granules forming a cross-linked starch structure. The cross-linked starch/latex composition is rapidly mixed with the filler slurry, which increased filler aggregation. The treated filler is then added to the papermaking furnish prior to sheet making. The treated filler prepared by this process was easily retained in the web during papermaking and the filled sheets have a higher internal bond and tensile strength than filled sheets produced using the conventional addition of cooked starch to the furnish.
At no point do any of the above patents disclose a method for the rapid and irreversible fixation of anionic polymer latex dispersions on filler induced by the addition of hot water at a temperature higher than the Tg of the polymer latex used. Also, there are no references in the open or patent literature related to the continuous treatment of filler with latex, in which the filler slurry is mixed with the anionic latex in mixing vessels that can control the degree of latex fixation on the filler by simply blending it with hot water under controlled shear and mixing time.